Extrasynaptic GABA(A) receptors: their function in the CNS and implications for disease

Abstract

Over the past two decades, research has identified extrasynaptic GABA(A) receptor populations that enable neurons to sense the low ambient GABA concentrations present in the extracellular space in order to generate a form of tonic inhibition not previously considered in studies of neuronal excitability. The importance of this tonic inhibition in regulating states of consciousness is highlighted by the fact that extrasynaptic GABA(A) receptors (GABA(A)Rs) are believed to be key targets for anesthetics, sleep-promoting drugs, neurosteroids, and alcohol. The neurosteroid sensitivity of these extrasynaptic GABA(A)Rs may explain their importance in stress-, ovarian cycle-, and pregnancy-related mood disorders. Moreover, disruptions in network dynamics associated with schizophrenia, epilepsy, and Parkinson's disease may well involve alterations in the tonic GABA(A)R-mediated conductance. Extrasynaptic GABA(A)Rs may therefore present a therapeutic target for treatment of these diseases, with the potential to enhance cognition and aid poststroke functional recovery.

Figure 1. Tonic inhibition is mediated by extrasynaptic GABA_A receptors

The dendrite/soma of a neuron receives a constant barrage of synaptic drive from glutamatergic and GABAergic terminals. The astrocytes that closely intermingle with these structures sense the release of these neurotransmitters as well as modulating their levels within the extracellular space. Vesicular GABA release from GABAergic terminals as well as non-vesicular GABA release from other sources interacts with GABA uptake mechanisms to set the ambient GABA levels within the extracellular space. I. Phasic inhibition: GABA molecules are packaged into synaptic vesicles within the GABAergic terminal. Once released, GABA rapidly diffuses across the synaptic cleft to occupy synaptic GABA_ARs that can exist in various subunit compositions. The low affinity of synaptic receptors means that although the synaptic cleft concentration is high (1–10 mM) the GABA molecules only occupy the receptors for a very short duration. Brief GABA_AR occupancy is further ensured by the rapid removal of GABA from the synaptic cleft (<1ms) due to diffusion and active binding and uptake by GABA transporter proteins (GAT-1) located in the presynaptic axon terminal. The resulting brief postsynaptic conductance change (white trace) is characterised by a fast rising and slow decaying waveform that can vary in duration depending upon the subunit composition of the synaptic GABA_ARs and the transmitter profile within the cleft. II. Tonic inhibition: The low resting ambient GABA levels present in the extracellular space are able to activate high-affinity extrasynaptic GABA_ARs to generate a persistent conductance (Cl- and to a lesser extent HCO_3⁻) that is responsible for generating tonic inhibition (noisy white trace) in a number of neuronal types. III. Ambient GABA levels: The precise mechanisms for regulating ambient GABA levels within the brains extracellular space is beginning to be elucidated and involves an interplay between the level of vesicular GABA release, the stoichiometry of the GABA transporters (GAT-2 and BGT-1 in astrocytes and GAT-1 in axon terminals) and other forms of non-vesicular GABA release such as GABA permeation through bestrophin channels. Ultimately, it is the level of ambient GABA that leads to the activation of extrasynaptic GABA_ARs in the soma/dendrite and even axonal membrane that generate the tonic inhibition.

Figure 2. Pharmacological strategies for altering the tonic conductance

A number of clinically relevant drugs are available that are known to alter the tonic conductance via a variety of direct and indirect targets. Here we illustrate a number of these targets situated within the principal neuronal and non-neuronal compartments of the brain. Although it was originally thought to be a GABA mimetic, the mechanism of action for Gabapentin is currently unclear, but the drugs ability to increase ambient GABA levels in the brain could reflect an alteration in GABA synthesis or release. Gabapentin is currently prescribed for the treatment of partial-onset seizures in adults and the elderly as well as a combination therapy for alcohol withdrawal and for sleep disorders. Tiagabine is a GABA transporter blocker acting predominantly on GAT-1 in nerve terminals leading to raised ambient GABA levels. This drug is prescribed for the treatment of partial seizures as well as generalized anxiety disorders/panic disorders. Other GABA transporter blockers such as SNAP-5114 are more selective blockers of GABA uptake in astrocytes, but these also lead to enhanced ambient GABA levels. Although bestrophin-1 channels could be an alternative nonvesicular source of GABA release, blockade of these channels by NPPB (5-nitro-2-(3-phenylpropylamino) benzoic acid) has been reported to both increase(Rossi et al., 2003) and decrease tonic inhibition(Lee et al., 2010) onto cerebellar granule cells. Irreversible block of GABA transaminase with the prescription drug Vigabatrin represents another strategy for raising ambient GABA levels. Vigabatrin has been used for the treatment of refractory complex partial seizures and infantile spasms but is currently not favoured due to visual field loss in some adults and children. More direct mechanisms for altering tonic inhibition involve orthosteric and allosteric interactions with extrasynaptic GABA_ARs. For example, the orthosteric agonist THIP or gaboxadol will selectively activate δ-GABA_ARs and, therefore, promote non-REM sleep. DS-1 is a newly developed agonist that has greater selectivity for δ-GABA_ARs than THIP, but its clinical benefit has yet to be established. Inverse agonists such as L-655,508 are currently being used to block the current generated by α5-GABA_ARs with the general objective to being used as cognitive enhancers. Allosteric modulators such as neurosteroids also offer a mechanism for more directly enhancing tonic inhibition. One such drug, Ganaxolone, is currently being developed for the treatment of drug resistant forms of catamenial epilepsy. It may also be possible to enhance or reduce tonic inhibition with Finasteride that blocks neurosteroid synthesis and XBD173 that enhances neurosteroid synthesis via the mitochondrial 18 kD translocator protein TSPO. It is also possible that the β subunit isoform identity may provide a means for selectively modulating tonic inhibition as the preferred β partner is the β2 subunit (Belelli and Lambert, 2005; Belelli et al., 2005; Herd et al., 2008) for α4βδ subunit-containing GABARs in the thalamus and dentate gyrus of the hippocampus. Any future development of β-subunit-dependent phosphorylation drugs could be useful in this regard.